1. Field of the Invention
The present invention relates to an electric parking brake control apparatus which is provided on a vehicle such as a motor vehicle.
2. Related Art
An electric parking brake drives a parking brake for holding a vehicle when it is parked or stopped by using an electric actuator such as a motor.
According to the electric parking brake, since a driver of the vehicle can operate the parking brake by an electric switch, labor involved in operating the parking brakes can be reduced compared with a hand-operated lever or foot-operated pedal which is generally used.
In addition, in the electric parking brake, the addition of a “hill hold (hill holder)” function has been proposed. The hill hold function is a function to prevent the unintentional movement of a vehicle by automatically activating the electric parking brake when the vehicle is stopped on a slope.
A braking force needed in the hill hold function differs depending upon the inclination (gradient) of a road surface or slope on which the vehicle is stopped, and for example, in order to prevent the vehicle from starting to move on a steep slope, a large braking force needs to be set. However, in the event that such a large braking force is generated at all times, the load to be borne by the electric parking brake apparatus is increased, and energy necessary for driving the apparatus is also increased.
Therefore, in the conventional electric parking brake apparatus, there is known one in which an acceleration (G) sensor as an inclination sensor for detecting the inclination of a road surface is installed so that the braking force is changed according to an output from the inclination sensor (for example, refer to JP-A-2004-142517).
In the case of using the G sensor as the inclination sensor, however, since deceleration G or deceleration G attributed to pitching or the like of the vehicle is detected while the vehicle is being decelerated or immediately after the vehicle is stopped, the accuracy with which inclination is determined is deteriorated. As a result, there is caused a time lag of the order of 2 seconds, for example, before an inclination can be determined accurately after the vehicle is stopped.
Therefore, there has been proposed to determine an inclination by comparing a change of a vehicle speed sensor immediately before the vehicle is stopped with a longitudinal G of the vehicle according to the G sensor. According to this proposal, an inclination can be determined within a short period of time after the vehicle is stopped. However, it is generally difficult to detect low vehicle speed with the vehicle speed sensor Therefore, in the case that the vehicle is stopped after transferring from no low-speed running to low-speed running, it is possible to determine inclination accuracy with the vehicle speed sensor, meanwhile, in the case that the vehicle is stopped again after the vehicle runs again in low-speed without experiencing no low-speed running, it is impossible to determine inclination accuracy. Therefore, in the event that the vehicle repeats a low-speed running and a stopping as when it is involved in a traffic jam on a slope, there maybe a case where it becomes difficult to secure the inclination determination accuracy.
In addition, in the case of a vehicle with a manual transmission it is expected to prevent the vehicle from moving in the event that the engine stalls.